Capacitively coupled plasma reactors are used in fabricating semiconductor microelectronic structures with high aspect ratios. Such structures typically have narrow, deep openings through one or more thin films formed on a semiconductor substrate. Capacitively coupled plasma reactors are used in various types of processes in fabricating such devices, including dielectric etch processes, metal etch processes, chemical vapor deposition and others. Such reactors are also employed in fabricating photolithographic masks and in fabricating semiconductor flat panel displays. Such applications depend upon plasma ions to enhance or enable desired processes. The density of the plasma ions over the surface of the semiconductor workpiece affects the process parameters, and is particularly critical in the fabrication of high aspect ratio microelectronic structures. In fact, a problem in fabricating high aspect ratio microelectronic integrated circuits is that non-uniformities in the plasma ion density across the workpiece surface can lead to process failure due to non-uniform etch rates or deposition rates.
A typical capacitively coupled reactor has a wafer support pedestal in the reactor chamber and a ceiling overlying the wafer support. The ceiling may include a gas distribution plate that sprays process gas into the chamber. An RF power source is applied across the wafer support and ceiling or wall to strike and maintain a plasma over the wafer support. The chamber is generally cylindrical, while the ceiling and wafer support are circular and coaxial with the cylindrical chamber to enhance uniform processing. Nevertheless, such reactors have non-uniform plasma density distributions. Typically, the radial density distribution of plasma ions is high over the center of the wafer support and low near the periphery, a significant problem. Various approaches are used to control the plasma ion density distribution so as to improve process uniformity across the wafer or workpiece surface, and at least partially overcome this problem.
One such approach is to provide a set of magnetic coils spaced circumferentially around the side of the reactor chamber, the coils all facing the center of the chamber. A relatively low frequency sinusoidal current is supplied to each coil, the sinusoidal currents in adjacent coils being offset in phase so as to produce a slowly rotating magnetic field over the wafer support. This feature tends to improve the radial distribution of plasma ion density over the wafer support. Where this approach is employed in reactive ion etching, it is called magnetically enhanced reactive ion etching (MERIE). This approach has certain limitations. In particular, the strength of the magnetic field may need to be limited in order to avoid device damage to microelectronic structures on the semiconductor workpiece associated with the strength of the magnetic field. The strength must also be limited to avoid chamber arcing associated with the rate of change of magnetic field strength. As a result, the total MERIE magnetic field may need to be substantially reduced and therefore may face substantial limitations in plasma ion density uniformity control.
Another approach is called configurable magnetic fields (CMF) and employs the same circumferentially spaced coils referred to above. But, in CMF the coils are operated so as to impose a magnetic field that extends across the plane of the workpiece support, from one side to the other. In addition, the magnetic field rotates about the axis of the wafer support, to produce a time-averaged magnetic field that is radial. This is all accomplished, in the case of a reactor having four side-by-side coils, by furnishing one D.C. current to one pair of adjacent coils and a different (or opposite) D.C. current to the opposite pair of adjacent coils. The coils are switched to rotate this pattern so that the magnetic field rotates, as mentioned above. This approach is vulnerable to chamber or wafer arcing problems due to the abrupt switching of the CMF magnetic fields, and therefore the magnetic field strength must be limited. As a result, in some applications the magnetic field cannot be sufficient to compensate for plasma ion density non-uniformities produced by the reactor.
Thus, what is needed is a way of compensating for plasma ion density distribution non-uniformities more efficiently (so that the magnetic field strength can be less) and with less (or with no) time fluctuations in the magnetic field.